Dicyandiamide is a well known nonpolymeric amide. If has found use in many applications, including fertilizers, pharmaceutical products, explosives, soldering compositions and fire-proofing compounds. Resin-like products can be produced by reacting dicyandiamide with formaldehyde to form a methylol derivative, as taught by Kitakawa, in the Journal of the Society of Chemical Industry, (Japan), 45, 416-419 (1942).
Son et al., in U.S. Pat. No. 3,732,286, taught acylguanidine catalyzed dicyandiamide and dicyandiamide type curing agents for epoxy resins. One of the many materials encompassed by the Son et al. curing agent general formula would include monomethylol dicyandiamide. The acylguanidine is used to lower the reaction temperature of cure. These materials are useful as potting resins, coatings and adhesives. Simms et al., in U.S. Pat. No. 3,759,914, in a patent similar to Son et al., taught a wide variety of amide catalyzed amine curing agents, for epoxy resins, to provide compositions useful as impregnants, potting and encapsulating compositions, powder coatings, surface coatings and in laminates and fillers, but primarily as adhesives for metals. The amine curing agents used include dicyandiamide, monomethylol dicyandiamide, benzoguanamine, melamine, adipamide, isophthalyl diamide and triaminomelamine, among twenty-five others listed. These compositions used from 0.1% to 300% amide catalyst based on the weight of amine curing agent, to lower the reaction temperature of the curing agents. Some of these epoxy compositions would still require extended heating times, and others would provide soft materials upon cure. Simms et al. lumps all of these amine curing agents together and recognizes no problems with dicyandiamide or any of the others in the manufacture of epoxy resin impregnated glass cloth laminates for multilayer printed wiring boards, where very strict military specifications are applicable in the areas of moisture resistance and cosmetic defects.
Additional uses for dicyandiamide have been found, for example, as a curing agent for phenol-aldehyde resins, as described by Baldwin et al., in U.S. Pat. No. 2,801,672, and as a curing agent for epoxy resins used in the preparation of glass cloth prepregs for printed circuit boards, as described, for example, by Gause et al., in U.S. Pat. No. 3,895,158. In Gause et al., a laminate is formed by disposing a resin impregnated layer of cellulose fiber paper between layers of glass fiber fabric sheets impregnated with a resin comprising epoxy resin and dicyandiamide, and then heat-pressure bonding the layers together.
However, dicyandiamide is relatively insoluble in epoxy resins at room temperature, and has limited solubility in low boiling organic solvents as well as water, which are commonly used to dilute the epoxy resins. Thus, there are limits on the amount of epoxy solids that can be used in the impregnating solution. Also, the large amount of solvent used to solubilize the dicyandiamide, results in long heat treating times, in order to remove all of the solvent so that no gas bubbles are formed in laminates impregnated with the epoxy-dicyandiamide system. This large amount of solvent is completely evaporated in processing, and poses a very substantial economic loss.
In addition, straight epoxy-dicyandiamide systems, if not carefully controlled, can precipitate dicyandiamide crystals during B-staging. These crystal formations are randomly disposed, and can be seen at the laminate surface. The crystals have a needlelike shape, ranging from 1 mm. to 10 mm. in length. They are in fact potential air pocket voids. During final cure to the C-stage, crystal formations which are actually voids occur due to the reabsorption during press lamination of dicyandiamide crystals which initially came out of solution at the prepreg stage. This crystallization and void formation leads to lowering of the moisture resistance of laminates impregnated with the epoxy-dicyandiamide system, due to the resin starved areas at the crystallization sites.
These problems have led to a search for a curing agent that has increased solubility in epoxy resins and water, allowing high solids content and use of less solvent, and which requires less heat treating time and also resists crystallization during B-stage prepreg heating and high temperature-pressure laminate molding of impregnated glass cloth laminates.